Distribution of Moths of the Sub-family Bistonince. 295 



assumed that peculiar state of sexual dimorphism before it 

 reached Japan. Now this observation brings it into con- 

 tact with B. comitata and its Indian relative B. regalis, whence 

 we glean that like many another member of Oriental stream 

 Biston overflowed from the Chinese area. 



Very soon, indeed, one offshoot pressed into regions with 

 climate so propitious that it waxed great, and part attained 

 such imposing proportions for a Geometrid that the name 

 B. robustum applied to it is no misnomer. So nearly confined 

 to the Japanese area was this section that, when in the end 

 Japan was separated from Corea and China, within its limits 

 was included the insect. But it was not so with the continental 

 stock. Leaving many detachments behind, some of which in 

 turn threw off colonies into Japan, that host spread widely, 

 ever seeking genial climes and as it gained them ever growing 

 larger. Finally it passed into India through the passes and 

 gorges of the Brahmaputra valley. There, limited only by its 

 predilections for trees of more temperate leanings, it reached 

 a size and appearance so noteworthy as to warrant the name 

 B. regalis. Its relatives in the ' old country ' attained no such 

 dimensions, and perforce have to remain contented with the 

 less presumptious title B. comitata. 



However, not all of the prototypical form was fortunate 

 enough to strike in these directions of novel and stimulating 

 conditions. Part of the original stock made a powerful thrust 

 to the west over the Great Central Plateau which then was 

 much better watered and not so relatively high as now, the 

 great uplift culminating in the huge tableland of to-day not 

 occurring until later. Soon, as it progressed, it was crowded 

 to the south-west, due western advance becoming impossible 

 owing to the enormous Central Sea ; thence its way was across 

 North Persia, through Armenia to Asia Minor. Onward, 

 without cessation, always greedy of new ground, it journeyed 

 into Balkan Area which then was one with Asia Minor. Unlike 

 what obtains to-day, direct advance then, into and across 

 Central Europe, was as yet impossible. The country about 

 the Danube was too impeded ; although quickly to vanish, the 

 old lakes and morasses yet beset the land. Necessarily, our 

 insect drove west across the future Balkan Peninsula to South 

 Italy and Sicily and thence across the Mid-Mediterranean 

 Subcontinent to North Africa, whence it once more invaded 

 European soil by crossing into Spain from Morocco, for the 

 Straits of Gibraltar had noi: then appeared. 



Immediately a slow northward movement was initiated, 

 and about the same time advance from the Balkans became im- 

 possible, and thus two streams were striving to occupy Europe, 

 no doubt in those warmer days with complete success. 



Scarcely was their journey thus apparently ended when 



1917 Sept. 1. 



